It is recognized that a wide variety of configurations of perimeter security fencing are required in order to service the many terrains to be followed, shapes to be enclosed and fence types utilized. For example, it is well known that existing taut-wire security systems provide a formidable barrier using multiple, closely spaced, horizontal barbed/razor wires. Such systems may provide for detection capability, wherein movement of one or more of the wires may “trip” a motion sensor and provide an indication of an attempted intrusion. Such a detection capability may be provided so long as the system is meticulously installed in flat and level locations and is operating in moderate weather conditions
Cost factors normally result in taut-wire systems being configured to provide a series of straight sections about 160 feet long between anchor posts with sensor posts located centrally between anchor posts. Further, intermediate taut wire support devices and posts may be located at 10-foot intervals between the anchor and sensor posts. Horizontal barbed wires, spaced vertically a few inches apart, are stretched, typically with about 70 pounds of tension, between the anchor posts. The normal quantity of these horizontal taut wires is 30 or more resulting in overturning forces of more than 2000 pounds at each anchor post. Accordingly, the anchor and corner posts for these systems require massive and expensive structural concrete foundations to resist the high side loads imparted on them by the tension forces of the horizontal taut wires.
Such existing taut wire systems also have a very limited capability to traverse vertical grade changes, and, due in part to the length of the taut wire runs, experience significant changes in taut wire tension due to temperature variations and support post movements caused by effects such as frost heave. Such variations are compensated for by using mechanical and/or electronic design features within the sensor mechanisms. These compensating mechanisms may compromise the validity of the detection mechanism by slowing the response of, and/or desensitizing, the motion sensors attached to the wires.
It is also well known that horizontal taut wires are compromised by ice and snow build-up causing taut wire systems to generate false alarms in winter conditions. Many existing systems will only work with the sensors installed in a vertical position extending between the horizontal wires and many existing sensors are also subject false alarms generated by electromagnet and radio frequency interference and suffer sensitivity changes due to temperature changes.
Detection of attempts to breach the fence in existing taut wire systems are typically localized to within the 160 feet between anchor posts. Detection of intrusions at or near the anchor posts can only result from the fracture of anchor tabs to which the taut wires are attached. These tabs are deliberately designed to break when the vertical force on the taut wire, as generated by an intruder or other means, exceeds the design fracture limit of the tab. Activation of the system by this means typically destroys the anchor tab and in most cases destroys the sensor thus requiring significant repairs to the system following a detected intrusion.
The sensitivity to motion of each horizontal taut wire in existing systems varies tremendously along the length of the taut wires between anchor posts and sensor posts ranging from reasonably high near the sensor post(s) to virtually nothing near the anchor posts. In order to maintain some semblance of uniform sensitivity on these known systems it is often required that substantial overlaps are provided at the anchor points of fence sections and at the fence corners.
Areas of a perimeter not suited to the installation of taut wire have been at least partially protected using microphonic security systems. Such systems may utilize the triboelectric effect of some coaxial cables to provide intrusion detection when the cables are attached to, for example, chain link or other fences. Accordingly, such systems can follow can follow most terrain variations. The coaxial sensor cables in these systems are detecting audible sounds, normally centered on the 400 Hz to 800 Hz range, These are secondary sounds generated by movements of the fence fabric to which the cables are attached.
These systems utilize various features of the incoming noise in the 400 Hz to 800 Hz range such as amplitude, duration and pulsing to try to determine if a human intrusion is taking place. Typically these systems will register an intrusion alarm when the detected secondary frequency amplitude exceeds a predetermined threshold for a predetermined time or if the amplitude threshold is exceeded for a shorter time a set number of times over a predetermined timeframe such as 4 to 10 times per minute. Using such determinations based upon secondary frequency sound sources leaves the system prone to misdiagnosis of such frequency as human intrusion when in fact they are generated by environmental inputs such as wind and rain.
However, the audible signals in the described 400 Hz to 800 Hz are secondary signals generated by different primary higher, lower and equal frequencies of motion applied to the fence which cause the metallic components of the fence to impact each other thus generating the aforesaid secondary frequencies. It has further been found that while standard galvanized wire chain link fences generate these high amplitude secondary frequency sounds, in response to fence fabric movement, vinyl coated chain link fence fabric generates such a low amplitude of secondary frequency sounds that the existing systems do not work with such fence fabrics. These secondary frequencies may also be generated by many extraneous environmental effects, man-made vibration sources (e.g., heavy traffic) as well as by human intrusion. The determination as to if a signal is generated by human intrusion or an environmental effect is problematic at best.
Stated otherwise, while these systems have a reputation for reasonable detection capability in clear calm environmental conditions, inclement weather such as rain, hail and wind, often generates false alarms in such systems. The technique currently used to eliminate these false alarms is to detect the adverse conditions and to reduce the system sensitivity such that the systems ignore the environmental conditions. In most cases this reduction of sensitivity will render such systems inoperable during poor weather conditions.
Further a trained intruder may defeat a microphone security system by circumventing other built in false alarm rejection means. Often these microphonic systems utilize a signal count method of rejecting high magnitude, apparently spurious, signals. The system will allow a predetermined number of short duration high magnitude signals from the fence within a preset time limit without generating an alarm. If the system is pre-set to allow 3 or 4 such signal inputs before tripping the alarm then an intruder can make a limited number of attacks (e.g., two) on the fence (such as cutting the chain link fabric) and then wait a suitable time (usually about sixty seconds) for the system to reset at which point more attacks can be made. A few minutes of such specific attacks can allow an intruder to gain entry.
The two types of systems described thus far are typical of the many different types of system used on high security perimeter fence lines. In addition, there are many different technologies used such as infrared beams, microwave beams, electrostatic fields both above and below ground. Any of these systems may be deployed along a perimeter and usually require one or more of the other systems in an attempt to cover the insensitivities or environmental failings of each other.
Typically on a large high security perimeter there are two parallel fences with a wide no-mans-land area between them. Each fence will be protected by a different type of security system while the gap between the fences may be protected by some type of volumetric security system. This results in a complexity of high maintenance equipment with suspect reliability that relies extensively on human supervision, especially in inclement weather conditions and covers a large area of real estate.